Piedmont Plateau of Georgia. — Watson. 20l. 
and are associated with similar rocks continuously into South 
Carolina. 
In the western section of the plain, near the Alabama line, 
somewhat extensive belts of very dark-colored, thinly banded 
hornblende gneisses or amphiboly tes are met. [Plate XVII, 
figures I and 2.] They are strongly contrasted with the gran- 
itic gneisses with which they are intimately associated. In thin 
sections, hornblende of a greenish-brown and blue color makes 
up half the rock or more. Plagioclase is the prevailing feld- 
spar, but orthoclase and quartz in variable amounts always ac- 
company it. Epidote is a frequent characteristic accessory. 
On general textural and structural grounds, the granitoid 
rocks are grouped and separately treated under three general 
headings : (a) The even-grained, normal, massive granites, 
(b) porphyritic granites, and (c) granite-gneisses.* The 
relationships of the three phasal aspects of the granite rocks 
are established in the succeeding paragraphs. 
(a) the even-grained, normal, massive granites. 
Granites of superior quality and variety, well-suited for 
general building and monumental work, have long been known 
in Georgia. Their extent, distribution, physical, mineral and 
chemical characters, however, are almost entirely unknown, 
since no systematic geological work has been undertaken. Un- 
til quite recently, the famous Stone mountain light-gray bio- 
tite-bearing muscovite granite was the only type of Georgia 
granite known outside the state. Within the past few years, 
however, several areas yielding a high grade monumental gran- 
ite have come somewhat prominently into favor in some sec- 
tions of the United States. 
PETROGRAPHY. 
With two exceptions, all the granites are biotite granites. 
Muscovite is prevailingly present in variable amounts, and 
very prominent as an accessory in some places ; while horn- 
blende fails entirely. They are described under the following 
types: The Oglesby-Lexington blue-gray; the Elberton-Ech- 
ol's Mill light-gray; the Campbell-Coweta-Meriwether coun- 
ties' medium blue-gray ; the Stone mountain light-gray. 
• The term granite-gneiss is nsed in this paper to denote gneisses derived 
from massive granites by met.iniorphism, and therefore igneous in origin, as 
contrasted with those gneisses of known sedimentary origin. 
